Final answer:
The correct verb form to fit into the sentence is 'bitten', indicating a past continuous action.
Step-by-step explanation:
The student was looking for the proper tense of the verb to bite, fitting into the context of the past continuous experience of being disturbed by mosquitoes, the correct form is bitten. The sentence should read: He could not sleep because he was bitten by mosquitoes. This verb form indicates that the action of being bitten by mosquitoes was ongoing in the past and affected his ability to sleep.